[ImperialCrown]

Welcome to Allpar. It should be covered under the new vehicle warranty and you should call the dealer to make an appointment with the service department.
If you can demonstrate the noise to the service advisor, shop foreman or the technician who will be working on it, that would be a good starting place for finding and fixing the issue. Happening 'sometimes' could be a problem as the noise really has to be present and repeatable in order to first diagnose it and then verify that it is gone afterwards. If you can 'make' it do it or know under what conditions it does it, that is important.
I didn't find any TSB's addressing this noise on 2012 WK's.

 

[chuzz]

It may be the seat belt anchors. I had a similar problem with my Caliber and it was repaired under warranty.

 

[beepshaw]

Just after getting my new 2012 G.C., I went to vacuum all the carpets and when I was in the rear seat area, I found that the left bolt, on the right rear seat had been only partly installed cross threaded from the factory. After removing it, I examined it for re-useable condition, removed the right bolt from its location, swapped the 2 and re-installed both, with no problems. IF....... you're not sure where the noise is coming from, I had a rattle that got worse and worse over time. I eventually found that the noise was coming from the extra roof glass panel behind the sunroof. After loosening the 6 screws that hold the panel in place, repositioning the panel slightly and re-tightening them all, - problem solved. Apparently this has been an ongoing problem with this panel. I could have had the dealer check it out, but before wasting my time having them look for it...DIY.